1. Field of the Invention
This invention relates a PC switching device which is installed between a keyboard (hereinafter KB) and personal computers (hereinafter PC's) when a plurality of PC's are operated by a single KB, and particularly relates a PC switching device which can be applied to various kinds of PC's, some of which have an electric power control method different from the others, and which is able to prevent the voltage of the electric power supplied to the KB from dropping and the reverse-current of the electric power from flowing.
2. Prior Art
Recently, there are many offices each of which has a plurality of PC's at the office, because of the development of the client-server system and an increase in the number of the power-users who simultaneously operates a plurality of PC's, but wide spaces are required, because each PC requires its own man-machine interface equipment (a keyboard, a pointing device, etc). Note, equipment other than a keyboard and a pointing device, that is, display equipment such as a CRT also belongs to a man-machine interface equipment, but in this specification, the display equipment is not treated as a man-machine interface equipment, because it contains its own electric power system, and does not receive electric power from a PC.
However, man-machine interface equipment is seldom operated simultaneously, and a power-user has to change working positions, where man-machine interface equipment corresponding to PC's are set, when the power-user uses a plurality of PC's.
FIG. 1 is a diagram showing connections between one set of man-machine interface equipment and a plurality of PC's. A PC switching device is installed between the one set of man-machine interface equipment which receives electric power from a corresponding PC, that is, a KB 111 and a mouse 112, and a plurality of PC's 121, 122, 123, and 124.
PC's can be divided into two types, that is, one type is a PC (hereinafter PC-A), the power of which is controlled by a power control key installed on a keyboard, and another type is a PC (hereinafter PC-B) power of which is not controlled by a power control switch on a keyboard, and is controlled by a switch installed on the PC itself.
Keyboards can be into two types, that is, one type is a keyboard (hereinafter KB-A) which has a power control key and another type is a keyboard (hereinafter KB-B) which has no power control key.
Therefore, there are four ways of connecting between PC's and KB's as follows.
1. PC-A is connected to KB-A.
2. PC-A is connected to KB-B.
3. PC-B is connected to KB-A.
4. PC-B is connected to KB-B.
FIG. 2 is a diagram showing connections between PC-A 21 and KB-A 22, and KB-A 22 contains not only KB-PSW 221, but also a key-matrix 222 and a keyboard CPU 223.
Note, one terminal of KB-PSW 221 is connected to a power supply terminal (hereinafter *PSW) of PC-A 21 which supplies a power to man-machine interface equipment through a reverse current protecting diode 224, and its other terminal is earthed. The key-matrix 222 is connected to a serial interface terminal of PC-A 21 (hereinafter SI/F) through the keyboard CPU 223, and the another terminal of KB-PSW 221 is connected to the serial interface terminal.
FIGS. 3A and 3B are flowcharts of a power on/off routine executed in PC-A. FIG. 3A is a flowchart of an off-state management routine, and FIG. 3B is a flowchart of an on-state management routine.
If KB-PSW 221 is pressed when PC-A 21 is in a power-off state, the off-state management routine is executed. A voltage drop at the *PSW terminal of PC-A 21 is detected at step 311, and PC-A 21 is turned on at step 312 so that the *PSW terminal is pulled up to, for example, +5V, and begins to supply power to the man-machine interface machines, and PC-A 21 stops to monitor the voltage of the *PSW terminal.
A press of KB-PSW 221 is transferred to the SI/F of PC-A 21 through the keyboard CPU 223 because monitoring of the voltage of the *PSW terminal is interrupted when the power-source of PC-A 21 is in an on-state, and the on-state management routine is initiated. In the on-state management routine, it is determined whether or not the received code corresponds to the power control key code at step 321, and this routine is terminated after executing power control management such as displaying a window for controlling the power at step 322 when the determination at step 321 is affirmative. Conversely, when the determination at step 321 is negative, this routine is terminated after executing a task corresponding to the received code is executed at step 323.
When KB-A is connected to PC-A, PC-A can be turned on by KB-PSW 221 installed on KB-A and prepared to be turned off.
When KB-B is connected to PC-A, this connection is meaningless because the power of PC-A cannot be controlled.
Both KB-A and KB-B can be connected to PC-B, because PC-B can be controlled by a power control switch installed on the PC-B cabinet.
Therefore, it is necessary to install a PC switching device between the PC's and a KB in order to make it possible to connect any of type of PC to any type of KB.
The PC switching device and one set of man-machine interface equipment receive power from the PC which has the highest *PSW voltage among the PC's connected to the PC switching device and one set of man-machine interface equipments, and reverse current protecting diodes are applied to prevent a reverse-current from flowing from the PC with the highest *PSW voltage to the other PC's.
A general diode, however, has a forward voltage drop, for example, 0.25V, and the man-machine interface equipments can be normally operated when the voltage of the power source is within (5±0.25V). When the highest voltage at a *PSW terminal of one PC is 5V, the voltage supplied to the man-machine interface equipments through the PC switching device may be lower than the voltage at which the man-machine interface equipment are normally driven.
To avoid a forward voltage drop, a relay or a transistor (including a field effect transistor) can be applied as a reverse current protecting device.
A relay, however, is a pure mechanical device, and has a drawback, that is, it has a short life. A transistor must be turned on in order to flow current though the transistor, and it must also be turned off in order to interrupt current though the transistor because the reverse current cannot be interrupted when it is kept in on-state.
FIG. 4 is a circuit diagram of a conventional PC switching device which is applied to solve the above problem, and shows the case where one KB is connected to three PC's PC1–3 through a PC switching device 4.
The *PSW terminals of PC's PC1–3 are connected to an input terminal of the DC—DC converter 44 through three reverse-current protecting diodes 41–43 in parallel.
The voltage supplied from the *PSW terminal which has the highest voltage among the *PSW terminals of PC's PC1–3 is stepped up by the DC—DC converter 44, and the power with stepped up voltage is supplied to a micro-computer 40 installed in the PC switching device 4, and KB through a reverse-current protecting diode 45.
Therefore, following problems occur in order to connect both types of KB's to both types of PC's.
FIG. 5 is a diagram showing connections when PC-A and PC-B are connected to KB-B though the PC switching device, but it is impossible to control power of PC-A by KB-B because KB-B is not equipped with a KB-PSW. (Problem 1)
FIG. 6 is a diagram showing connections when PC-A's are connected to KB-A though the PC switching device, but it is impossible selectively to control power of specified PC's though all PC's can be turned on simultaneously. (Problem 2)
When a step-up circuit is installed in a PC switching device, it is necessary to install not only an inductor 46 in parallel to the DC—DC converter 44, but also condensers 47–49 to remove ripples at the input and output terminals of the DC—DC converter 44 and the cathode terminal of the reverse-current protecting diode 45. But the configuration becomes very complex, and the price of the PC switching device becomes high because the above elements are expensive. (Problem 3)
Furthermore, it is not avoidable that the efficiency of the DC—DC converter 44 deteriorates, because the DC—DC converter 44 uses a current-voltage converting method, and consumes more power than that consumed in the PC switching device 4 and the KB. (Problem 4)